Tom Buck on mon 12 jan 98
There was a bit of transmission garble (my head to the screen at 1 am) so
I am repeating the earlier post with the correction in Caps in the
penultimate paragraph (ie, 2nd last).
Tom Buck
---------- Forwarded message ----------
Date: Thu, 8 Jan 1998 00:47:03 -0500 (EST)
From: Tom Buck
To: clayart forum
Subject: glaze movement
Ouch & Groan! I feel like a one-ball juggler who confused up & down
and so dropped the ball.
My picture of what was going on between claybody and glaze
was reasonable, and the numbers in question were ballpark for the
glaze being talked about. But instead of seeing the glaze in tension,
I saw glaze in compression; hence, shivering instead of crazing.
Let's try a new picture. Imagine a test tile a million
millimetres long (ie, 1 kilometre in length). The tile has been
bisqued, and coated with glaze. But this time the glaze is afixed
only to one end and otherwise "floats" atop of the body. The body
expands 5.5 x10-6 units of length per unit of length per degree
Celsius (Centigrade) or 3 x 10-6 L/Lxdeg F.
Now, recall that the glaze doesn't exist until the ingredients
melt, coalesce, and combine into the macro- (huge) molecules, like
extremely tiny pieces of string, and which aggregate to form glass. So
there is no glaze expansion during the rise to fusion temperature, and no
contraction until the glaze solidifies. Let's say the glaze goes solid at
1020 C (1840 F) and we will cool it to 20 C (70F). And also note that the
solid glaze contracts at 8 x10-6 L/Lxdeg C (4.4 x 10-6 L/LxdegF).
Let's say that both body and glaze start to shrink at 1020 C
and do so uniformly til room temperature, 20 C, is reached.
The body (a 1 km tile) contracts 1,000,000 x 0.0000055 x 1000
or 5.5 x 1000 millimetres, 5500 mm, or 5.5 metres.
The glaze contracts 1,000,000 x .0.000 008 x 1000 or 8 x 1000 mm,
ie, 8.0 metres.
So, at one end of this imaginary tile there would be exposed an
unglazed length of 2.5 metres (8-5.5), a quite noticeable gap.
The same shrinkage occurs on a fired-pot as it cools. But because
the glaze melts on, and afixes to, the surface of the pot everywhere it
has been placed, the difference in shrinkage between body and glaze sets
up large stresses (forces that will break physical, chemical bonds). In
turn, these stresses will disrupt the continuity of the glass "strings"
at irregularities in the body/glaze interface (boundary layer). The glaze,
shrinking more, will crack and open up at these irregularities. We say the
glaze "has crazed".
A similar analogy would explain what happens when the glaze
contracts LESS than the body. Then the glaze bulges up from the boundary
layer, and pieces of the glaze crack off. We say the glaze "has shivered".
My apologies to all for getting it backwards in my earlier post. I
hope I got it right this time. Til later. Tom.
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).
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